Oooh, quite a find there. I assume that's in the final, right? If so, might be worth looking through some of the Chaotix protos that were part of drx's mass proto release when you're done; see if there's any leftovers in those that were cleaned up before the final release.

Looking at it, the first thing that comes to my mind is the S3&K ending sequence. Isn't that, like, the only other time we see the plane flying to the left of the screen? In fact, come to think of it, doesn't a larger piece of artwork of Sonic & Tails on the Tornado appear in Chaotix's good ending? Could this at one point have been intended as part of an ending sequence? Whatever it's for, I see the Chaotix art team actually bothered to fix the "SONIC" on the side - S3&K just mirrored the plane for the ending sequence since it's seen flying right in all other instances, so the letters were mirror-image in that game.

The screenshots above have a border drawn over the outermost pixels because .net is stupid and border style FixedSingle does that. I will be releasing my viewer later so you can use that for more accurate GIFs.

This really doesn't make a lot of sense to me. andlabs told me this was discovered in the final. How come we weren't able to find it in the 1207 proto? All art in that proto (to my knowledge) is uncompressed. Did they attempt to put Tails in after all? According to what andlabs has told me this was found in a section where a lot of the sprites used the 32x's scaling. It could be a potential alternate ending that was planned, or maybe something more or different entirely. The world may never know and it's such a shame Tails got scrapped. :/